1. Field of the Invention
The present invention relates to a moving image coding apparatus, and more particularly to a technique for selecting an optimal intra prediction mode when the H.264 coding standard is used.
2. Description of the Related Art
Various coding standards, such as Motion-JPEG, MPEG-1 and MPEG-2, have been established as techniques for high-efficiency coding of moving images. Manufacturers have been developing and marketing image capture apparatuses, such as digital cameras, digital video cameras, DVD (digital versatile disk) players, etc., which are capable of storing moving images using these coding standards. Accordingly, users are allowed to easily play back moving images using these image capture apparatuses, DVD players, personal computers, or the like.
Digitized moving images carry large mounts of data. Therefore, various coding methods for moving images capable of performing more efficient high compression than the above coding standards, such as MPEG-1 or MPEG-2, have been continuously researched and developed. Recently, a new coding algorithm called H.264/MPEG-4 Part 10 AVC (hereinafter referred to as the H.264 standard) has been standardized by the ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) and the ISO (International Organization for Standardization).
The H.264 standard requires a large computational complexity for coding and decoding as compared to the conventional coding standards, such as MPEG-1 and MPEG-2, but provides a higher coding efficiency. A system and process for computation processing using the H.264 standard are disclosed, for example, in Japanese Laid-Open Patent Application No. 2004-56827.
The H.264 standard includes a prediction method known as intra prediction for predicting pixel values within a given frame by using pixel values within the same frame. In this intra prediction, there are a plurality of intra prediction modes, which are selectively used. In this instance, an intra prediction mode suitable for an input image is selected to form coded data that has little deterioration even after being subjected to highly efficient compression.
As for the intra prediction, the H.264 standard provides nine intra prediction modes to improve the precision of prediction. An optimal intra prediction mode is generally selected from the nine intra prediction modes by tentatively executing all of the intra prediction modes for an input image and, based on a result of the tentative execution, finding an intra prediction mode capable of obtaining an optimal result.
The reason for employing such a selection method is described below with reference to FIG. 17. FIG. 17 is a diagram showing a frame, in which an outer quadrilateral denotes the entire picture and five inner quadrilaterals denote blocks to be subjected to intra prediction. Although blocks are actually set over the entire picture, only five representative blocks are illustrated for the sake of convenience of description. The arrows in FIG. 17 indicate the direction of prediction indicated by an intra prediction mode that has been determined to be optimal as a result of computation in each block. Thus, since various objects are present at the respective locations even within the same picture, different intra prediction modes may be determined to be optimal for the respective blocks, and more than one particular intra prediction mode may be selected. For such a reason, in performing intra prediction, the precision of prediction is obtained for all of the intra prediction modes in each block, and an intra prediction mode capable of performing optimal prediction is selected and designated in each block.
However, computation for all of the intra prediction modes in each block to select an optimal intra prediction mode from among them increases computational complexity in the H.264 coding process, thus resulting in an excessive increase in coding processing time or a wasteful consumption of electric power.